The Influence of Four Unnecessary Genes for Virulence on the Fitness ofErysiphe graminisf. sp.tritici

Abstract

Two isolates of Erysiphe graminis f. sp. tritici that differed in their host range were examined for possible evidence in support of Vanderplank''s stabilizing selection hypothesis. Isolate MO-10 was virulent and isolate MS-1 avirulent on wheat line CI 15880 and lines with the Pm2, Pm3a, and Pm4 resistance genes. Genetic analysis of the isolates showed one locus controlling each difference in host range. A mix of the two isolates was grown on cultivar Chancellor (on which both isolates were virulent) in a controlled environment and the relative proportions of each isolate were monitored over successive conidial generations. The frequency of MO-10 decreased and the frequency of MS-1 increased, indicating that MO-10 was less fit than MS-1 under these conditions (s.hivin.x = 0.24). However, when F1 progeny of the isolates were compared in 8-way mixtures, reduced fitness was found to have segregated independently of the identified virulence loci. The results indicated that the fitness of the progeny was not significantly affected by the unnecessary virulence alleles. No evidence was found to support the hypothesis of stabilizing selection.